In the manufacture of planar exhaust gas analyzer probes, such as customary lambda sensors based on thick-film technology, electrically insulated plated holes known as vias are used to connect, for example, printed conductors on different sensor levels or the front or back of a ceramic laminated composite.
If the material from which the ceramic green films are made is electrically insulating, e.g., an LTCC ceramic based on Al2O3 (where “LTCC” denotes a low-temperature co-fired ceramic), a hole is drilled or punched into the ceramic green film and filled with a paste filled with a conductive material such as silver or platinum, as is customary in the manufacture of printed conductors.
It is more difficult to produce an insulated plated hole, which is suitable for electrically poorly conductive film materials such as yttrium-stabilized zirconium dioxide.
In order to be able to implement an electrical plated hole in the case of ceramic green films containing yttrium-stabilized zirconium dioxide, from which an yttrium-stabilized zirconium dioxide ceramic is produced after sintering, the use of a platinum-containing paste, to which niobium oxide (Nb2O5) is added, has been described. The latter is able to compensate the yttrium oxide doping of the zirconium dioxide, resulting in a stoichiometry of the ion conductor “yttrium-stabilized zirconium dioxide” in the area of the via or the electrical plated hole, which is characterized by a relatively low electrical conductivity.
An additional technique for integrating electrically insulated plated holes in conductive, ceramic multilayered structures is to introduce a hole for the electrical plated hole into the green film(s), and then to coat or to wet this hole on the wall of the green film(s) delimiting the hole with an insulation paste and then to introduce the conductive material, i.e., the platinum-filled paste, for example, into the now insulated hole.
A disadvantage of the above-described method for producing the plated hole and of the previously produced plated holes is that at high temperatures, the Nb2O5 counter-doping is made partially ineffective so that the insulation resistance to the electrical plated hole is significantly reduced. Furthermore, the suction of two different pastes, first an insulation paste and then a conductive paste is very difficult, in particular in a multilayered composite of ceramic green films or a ceramic laminated composite (manufactured from it by sintering) because it is hardly possible to ensure that the insulation layer applied first to the inner walls is entirely free from cracks and thus reliably prevents the conductive paste from contacting the electrically conductive ceramic green films and the ceramic layers sintered from it.
Moreover, the insulating effect of known insulated plated holes is generally low at high temperatures in particular because insulating layers used previously have only low layer thicknesses due to their method of manufacture.
As a whole, it has thus far not been possible to ensure a reliable, high-resistivity insulation of a via using a ceramic laminated composite or a ceramic layer at high temperatures of use.